In recent years, standardization of 4th generation mobile communication has been underway, and various investigations have been conducted for the purpose of accomplishing the improvement of the downlink transfer rate. The downlink is communication from a base station to a mobile station. One method that can be envisioned for improving the transfer rate is that of widening the system bandwidth. However, the widening of the system bandwidth is restricted because of limited frequency resources. Given this, investigations are done regarding MIMO (multiple-input, multiple-output), which enables spatial multiplexing of a plurality of signals, using one and the same of the time and of the frequency, without widening the frequency bandwidth, as the practical technology. MIMO multiplexing technology is spatial multiplexing transmitting technology, whereby spatial multiplexing is done by providing a plurality of antennas for both transmitting and receiving sides and simultaneously transmitting different signal streams from the plurality of transmission antennas.
In 3.9G, to satisfy the requirement condition for a transfer rate of 300 Mbps (megabits/second), MIMO technology that provides four antennas and enables maximum four-fold multiplexing is used. In the 4th generation, considering the requirement for a 1-Gbps (gigabit/second) bandwidth, it is necessary to have spatial multiplexing up to a maximum of eight-fold, and related investigations are being conducted. In this case, methods for separating MIMO-multiplexed signals can be generally divided into two types: methods in which signal separation is done at the receiving side, and methods in which signal processing is done at the transmitting side.
The wireless channel in a mobile communication system is constituted by a large number of channels with different delay times. For this reason, at the receiving side, in addition to the desired signal to be transmitted and received, there are a large number of interference signals. The reception characteristics, therefore, are influenced by the influence of interference and the influence of noise.
Methods of separating multiplexed signals at the receiving side include MMSE (minimum mean square error) and MLD (maximum likelihood detection). With these methods, using the received signals of each reception antenna and the characteristics of the channels through which the signals pass, detection is performed of the desired signal to be transmitted and received, taking into consideration the influence of the interference and noise on the channels. The reception characteristics are relatively good. With WILD in particular, it is possible for all combinations of modulated signals that can be transmitted from the transmission antenna to select the one that has the highest probability of being the one that was transmitted. For this reason, MLD is the optimum receiving detection method, and achieves good transfer characteristics.
Transmitting equalization techniques such as ZF (zero forcing) and THP (Tomlinson-Harashima precoding) have been proposed as methods of signal processing at the transmitting side (refer to Non-Patent Document 1). Transmitting equalization techniques are techniques that suppress at the transmitting side the influence of interference received on the channel. Specifically, in transmitting equalization, the channel characteristics that are estimated for each reception antenna are fed back to the transmitting side. Then, at the transmitting side, the influence of interference on the channel is calculated based on that channel information, and the signal is processed prior to transmitting, so that it is possible at the receiving side to receive a signal in which the influence of interference has been cancelled out. Although the reception characteristics are deteriorated from those of a method that performs MIMO signal separation (separation of signals as the MIMO receiving side, hereinafter “MIMO separation”), the receiving processing is simple.